A typical scanning probe microscope is a device that may be used to measure the distance between the tip of a probe needle and a test sample surface, also referred to as the probe target. Such distance is determined by measuring physical quantities between the probe needle tip and the test surface. For example, the scanning probe microscope may measure the electrostatic capacitance and contact voltage between the probe needle tip and the test surface, or a tunnel current flowing through the probe needle tip and test surface. A scanning probe microscope can detect physical quantities, such as distance, in an extremely small region of the test sample surface. This region may have dimensions in the order of several nanometers.
Scanning probe microscope technology may be used in various applications. For example, a moving medium-type memory device that incorporates scanning probe microscope technology can record and reproduce one bit of information stored in an extremely small region. This region may have a diameter of about 30 nm.
In applications such as a scanning probe microscope or memory device, the probe and probe target must be capable of being positioned relative to one other with an error of less than the desired spatial resolution. For example, in a moving medium-type memory device, the probe and probe target must be positioned relative to one another with an error of less than the size of the region used to record one bit. The device must be able to position the probe or probe target with a positional error that is less than the dimensions of smallest possible region of the target surface it is desired to be resolved accurately. Accordingly, the precision with which the probe or probe target positioning is controlled affects how small a region can be accessed by such device. Moreover, since most positioning devices are closed-loop devices that include a position detection device, the accuracy with which the positioning device can position the probe target depends on the accuracy with which the position detection device can detect the position of the probe target.
In Piezo/Electrostrictive Actuators, pp. 94-102, Morikita Shuppan, (1990), K. Uchino disclosed various methods that can be used to implement a position detection device of the type required for positioning a probe target with a precision of the order of that described above. However, most of the devices disclosed by Uchino are physically large and therefore cannot be fabricated as part of an integrated circuit.
FIG. 1 shows a position detection device that is based upon capacitance and that can form part of an integrated circuit. A typical capacitance-type position detection device includes four capacitors that are arranged to form the AC bridge circuit 21 shown in FIG. 1. One of the capacitors forming the AC bridge circuit 21 includes the first electrode A fixed to the target stage 23, and includes the second electrode B fixed to the substrate 24 shown in FIG. 2. The electrodes A and B are oriented perpendicular to the direction of motion of the target stage. The remaining three capacitors of the AC bridge circuit are fixed capacitors. The position detection device detects a change in the relative position between the target stage and substrate from the variation in the capacitance of the capacitor formed by the electrodes A and B. Such variations in capacitance result from corresponding changes in the gap d between the electrodes A and B of the capacitor.
The differential output voltage between the output nodes of the AC bridge 21 depends on the change in the gap d between the capacitor electrodes A and B. This voltage is fed to the detection circuit 6, which includes the differential amplifier 36, the synchronous detector circuit 121 and the low-pass filter 20. The synchronous detector circuit 121 detects the amplified differential output voltage of the AC bridge. The low-pass filter 20 removes the high-frequency component of the signal generated by the synchronous detector to provide a direct current (DC) output signal that represents the relative displacement between the probe stage 23 and the substrate 24.
To improve position detection sensitivity in the electrostatic capacitance position detection device described above, the area of capacitor electrodes must be increased. Since the capacitor electrodes are disposed perpendicular to the major surfaces of the target stage and the substrate, this would require that relatively large appendages carrying the capacitor electrodes be affixed to the target stage and substrate. As a result, probe devices that use such position detection techniques must be relatively large. As such, they are not useful for such applications as integrated circuits.
What is needed is a position detection device that has a high detection resolution, an output signal that linearly represents position, and that can easily be made as part of an integrated circuit.